Light is presently being widely investigated, and to a lesser extent actually employed, in many applications as a replacement for electrical signals. Examples include optical communication, optical data transmission, and optical signal processing.
In all of these systems, the basic component is an optical waveguide. One of the simplest waveguides is referred to as a planar waveguide and has a sandwich structure wherein the center of the sandwich is a thin film having a height of the order of the propagating light wavelength and an index of refraction which is greater than that of the layers surrounding it. This makes possible total internal reflection which is the mechanism that causes the light to be confined within the waveguide.
Planar waveguides confine light in only one dimension, the height. It is also necessary for many waveguide devices to confine light in another direction, the lateral direction. This allows the guided light to be steered around curves and serves as the basis for a variety of proposed integrated optical circuits. Usually, confinement in the lateral direction is achieved by tailoring the device so that a larger effective refraction index exists over a three-dimensional region comprising the waveguide. Such waveguides are generally referred to as three-dimensional waveguides.
Much of the research effort directed to producing optical waveguides from semiconductor layers has been focused upon gallium arsenide and other III-V compounds. This is due to the fact that some of these materials are very versatile. In fact, all the important functions, including light generation, guiding, modulation and detection, have been achieved in gallium arsenide based materials.
One form of optical waveguide which has been fabricated with gallium arsenide is based upon the fact that conduction electrons or holes make a negative contribution to the dielectric constant, thus decreasing the index of refraction. Thus, an optical waveguide might be formed with the gallium arsenide substrate doped with a high electron concentration (n.sup.+) with a guiding layer of gallium arsenide deposited thereon which has a low electron concentration (n.sup.-). In such devices, index differences as large as tenths of a percent have been achieved and found to be sufficient to produce total internal reflection.
Gallium arsenide waveguides have also been produced from heterostructures, such as a single layer of GaAs deposited upon a Ga.sub.1-x Al.sub.x As substrate.
Despite the succes which has been achieved with gallium arsenide optical waveguides, such devices have had problems. One of these problems is the high propagation loss which results as light propagates through the light guiding layer. There is invariably a portion of the light which leaks into the substrate material which is lossy.
Another deficiency of previously existing gallium arsenide optical waveguides is their inability to guide light through sharp bends or direction changes without concomitant high loss.